This invention is related to a process for making necked nonwoven webs and laminates having more uniform basis weights and stretching properties in the cross direction.
Necked nonwoven webs, including necked spunbond webs, meltblown webs, combinations and the like, are often made using a process which is schematically illustrated in FIG. 1. A nonwoven, web 12 having a starting width A is passed in its machine direction between a first nip 16, which can be a first pair of nip rollers traveling at a first surface velocity, and a second nip 26, which can be a second pair of nip rollers traveling at a second surface velocity which is faster than the first surface velocity. The surface velocity difference between the first and second nips results in formation of a narrower (“necked”) nonwoven web 22 having a necked width C which is less than the starting width A.
The necked nonwoven web 22 generally includes fibers which are closer together and more aligned in the machine direction than the fibers of the starting nonwoven web 12, which can be more randomly aligned. The necking may be performed with the aid of heat applied below the melting temperature of the fibers, for instance, by placing an oven or other heat source between the first and second nips. The necked nonwoven web 22 may also be heat set, either during or after the necking process, so that the necked web becomes somewhat stable. A nonwoven web which is stable in the necked condition is said to be “reversibly necked”. A reversibly necked nonwoven web can be easily extended in the cross direction by applying a small extension force, and tends to return to its narrower, necked configuration when the extension force is released.
The starting nonwoven web 12 includes edge regions 13 and 15, and a central region 11. The necked nonwoven web 22 includes edge regions 23 and 25, and a central region 21. Because the necking causes the nonwoven fibers to become closer together and more aligned, without noticeably stretching or narrowing the individual fibers, the necked nonwoven web 22 generally has a higher basis weight than the starting nonwoven web 12.
As can be easily seen from FIG. 1, the nonwoven fibers in the edge regions 13 and 15 of the starting nonwoven web travel a greater distance between the first nip 16 and the second nip 26 of the necking process, than the fibers in the central region 11. This results in more extension of the web at the edges and increased fiber gathering and necking in the edge regions.
A second and independent cause of greater necking at the web edges results from the curvature of the web edges under tension. From biaxial stress analysis as shown in FIG. 2C, it can be shown that the web tension in the curved edge regions 13 and 15 creates a cross-directional stress field which is greater at the center region than at the edges. This cross-directional stress field counteracts the compressive forces which cause necking of the web, counteracting more forces at the center region 11 and less forces at the edge regions 13 and 15.
Consequently, the fibers in the edge regions 23 and 25 of the necked nonwoven web are generally more aligned and closer together than the fibers in the central region 21. As a result, the necked nonwoven web may be nonuniform in the cross direction, having a higher basis weight in both edge regions than in the central region, and having greater cross-directional extendibility in both edge regions than the central region.
Various techniques for producing necked nonwoven webs having improved cross-directional uniformity are described in U.S. Pat. Nos. 6,803,009 and 6,900,147, both issued to Morman et al. The disclosed techniques involve modifications to the necking process and/or material being necked. There is a need or desire for a necking process which achieves cross-directional uniformity in a self-regulating and self-controlling fashion without requiring modification of existing necking equipment or compositional modifications to the material being necked.